The Now TV app is now heavily based on the Peacock App (which was initially forked from the original Now TV app).
After looking through this thread, I was able to get a command working that will launch directly into a live channel in the Now TV app.
adb shell am start -a android.intent.action.VIEW -d 'https://tv.client.ott.sky.com/?deeplinkData=%7B%22serviceKey%22%3A%224061%22%2C%22type%22%3A%22LINEAR_CHANNEL%22%2C%22action%22%3A%22PLAY%22%7D' com.bskyb.nowtv.beta
That will launch Sky Max HD for example.
You need to set the following data and pass it in after https://tv.client.ott.sky.com/?deeplinkData= but it needs to be URL encoded
{"serviceKey":"4061","type":"LINEAR_CHANNEL","action":"PLAY"}
serviceKey is the ID you take from the URL on the NowTV web version like you do with Apple TV deeplinks.
Also as a bonus I also found it's possible to launch the Channel 4 live streams even though there is no option in the Channel 4 app on Android.
adb shell am start -a android.intent.action.VIEW -c android.intent.category.DEFAULT -d "https://www.channel4.com/now/C4" com.channel4.ondemand
That should launch Channel 4, you can change it to e4 for E4. However I find I have to quit the app before I change channels otherwise it just loads a grey screen.